Abstract
Three-photon ionisation cross sections of He(21S) and He(23S) have been measured by crossing a metastable helium beam (density approximately 3*103 cm-3) with a focused ruby laser pulse linearly polarised (frequency 14398.5 cm-1 and intensity 1011 W cm-2). The measured ionisation cross sections for singlets and triplets are respectively sigma s=(3.3+or-1.9)*10-80 cm6 s2 and sigma T=(3.0+or-2.2)*10-81 cm6 s2. Two-photon ionisation cross sections of He (21S) and He(23S) have also been measured by using the second harmonic generation (frequency 28797 cm-1 and intensity 1011 W cm-2) of the Q-switched ruby laser. The measured ionisation cross sections of singlets and triplets are respectively sigma s=(2.7+or-2)*10-40 cm3 s and sigma T=(1.5+or-1.4)*10-49 cm4 s. By using a new method the authors have calculated these cross sections for their experimental conditions. The agreement between the experimental data and the theoretical predictions is satisfying.